The invention relates to a radio receiver in accordance with the heterodyne principle or with the homodyne principle. The reception quality of a radio receiver is determined by a number of criteria, the most important of which are absolute signal sensitivity, adjacent channel selectivity and strong signal behavior. Strong signal behavior concerns mainly harmonic mixing, intermodulation and blocking.
Harmonic mixes are achieved with the formation of the intermediate frequency by mixing the harmonics of the input signals with the harmonics of the oscillator signal. Only one signal is necessary here.
The most important interference components of this type occur at the following frequencies of the interference signals: EQU f.sub.st =f.sub.in +1/2f.sub.z ; EQU f.sub.st =f.sub.in +2/3f.sub.z ; EQU f.sub.st =2(f.sub.in +f.sub.z).+-.f.sub.z
where f.sub.in is the reception frequency, f.sub.st the interference frequency, and f.sub.z the intermediate frequency.
Intermodulation interference is the mixing products of at least two signals that drop into the desired reception channel with the appropriate frequency constellation. The well known intermodulation interferences occurring most strongly result from second and third order intermodulation under the following frequency conditions: EQU 2nd order: f.sub.z =.vertline.f.sub.s2 .+-.f.sub.s1 .vertline.; f.sub.in =.vertline.f.sub.s2 -f.sub.s1 .vertline. EQU 3rd order: f.sub.in =.vertline.2.sub.fs2 -f.sub.s1 .vertline.; f.sub.in =.vertline.f.sub.s1 .+-.f.sub.s2 .+-.f.sub.s3 .vertline.
where f.sub.s1 to f.sub.s3 are the interference signal frequencies.
The determining factor for the interfering products are the non-linearity in the pre-amplifier and mixer stage, in conjunction with the degree of HF selection in the pre-amplifier.
The blocking behavior describes the reception behavior of the receiver for weak desired signals in the presence of strong in-band interference signals. This is the effect in which weak desired signals disappear in noise when strong interference signals occur in the reception band. This is the case, for example, when the gain is regulated down by a strong in-band signal, as is the case in FM radio receivers.
The main cause for reception problems is therefore non-linearities in the front end. These occur both in the active and in the passive components. The result is, in addition to the well known distortions in the pre-amp transistor and the (usually integrated) mixer, further distortions (of various type) in the varactor diodes used for tuning the RF band-pass filters.
Generally speaking, the problems with the agc concept of a classic FM radio receiver can be divided up and treated in the following main parts:
a) location of gain control in the signal path;
b) type and properties of the components controlling the gain; and
c) location and type of control signal generation.
Re. a) and b): all modern concepts use PIN diodes for control of the gain. More precisely however, it is a control of the signal attentuation in the signal path in front of the pre-amp transistor (location of signal attentuation).
Re. c): Wide-band agc has become general, but is however sometimes additionally controlled by various criteria.
The advantage of wide-band agc is that strong interference signals are attentuated before they cause, for example, heavy intermodulation or overload the varactor diodes or transistors. The drawback of wide-band agc is that the reception of weak desired signals (blocking) is impaired. Attempts are made to counter this drawback by control of the agc threshold.
Wide-band agc requires that the control signal be gained at points in the signal path with a sufficiently wide band-width. Generally, these are the mixer input (point B of FIG. 1) or the mixer output (point A of FIG. 1), which must be designed sufficiently wide band-width.
Whereas the band-width at the mixer input is predetermined by the RF band-width, it is possible to additionally influence the band-width at the mixer output (IF) and thereby the band-width of the agc loop. It is thus possible to achieve a restriction of the agc band-width.
The control of the agc threshold stems from the requirement to achieve an acceptable compromise between the requirement for good intermodulation behavior on the one hand and avoidance of strong blocking effects on the other.
For this purpose, the transmission of the wide-band agc signal from the output of the first signal detector 13 to the agc amplifier 15 is controlled by the AND GATE circuit 14 in accordance with FIG. 1, which shows the block circuit of the FM part of modern car radios. The control signal is the field-strength signal of the IF amplifier 11. This form of control of the wide-band agc only becomes effective (is switched through) when a desired signal of sufficient strength is present.
When a control system of this type is used, however, problem occurs when the receiver is tuned to an unoccupied desired channel. In this case, no control at all would be achieved in this way, with the result that an uncontrolled overload of the varactor diodes or transistors can occur, for example.
To avoid this, additional closed control loops or intervention means are provided. These are shown in FIG. 1. Via a second signal detector 16, a control signal is gained by Point C and supplied directly to the agc amplifier 15, thereby "bridging" the AND GATE 14.
Another possibility is to achieve the control signal via the third signal detector 12 (Point A) and feed of its output signal to the control line 17. With a suitable setting of the response threshold of the third signal detector 12, the AND GATE can be activated and the danger of overload avoided.
As a general principle, the blocking behavior problem occuring in known radio receivers could be largely avoided if an interference signal in the desired signal channel could be distinguished from a desired signal. In this case, a decision-making criterion would be available that permits switch-on of the agc only when the interference affects the desired channel. This would avoid the situation normal in the prior art that the reception of weak signals is disturbed by "unnecessary" controlling down of the gain (blocking).
Achievement of a control concept on this basis would however entail a new function unit permitting recognition of interference in the desired channel and generating an appropriate output signal.